Update Manifest docs

Author: tverghis

src/doc/src/reference/manifest.md

@@ -168,7 +168,7 @@ readme = "README.md"
 If no value is specified for this field, and a file named `README.md`,
 `README.txt` or `README` exists in the package root, then the name of that
 file will be used. You can suppress this behavior by setting this field to
-`"false"`.
+`false`.
 
 #### The `homepage` field